Angular exception: Can't bind to 'ngForIn' since it isn't a known native property

What am I doing wrong?

import {bootstrap, Component} from 'angular2/angular2'

@Component({
  selector: 'conf-talks',
  template: `<div *ngFor="let talk in talks">
     {{talk.title}} by {{talk.speaker}}
     <p>{{talk.description}}
   </div>`
})
class ConfTalks {
  talks = [ {title: 't1', speaker: 'Brian', description: 'talk 1'},
            {title: 't2', speaker: 'Julie', description: 'talk 2'}];
}
@Component({
  selector: 'my-app',
  directives: [ConfTalks],
  template: '<conf-talks></conf-talks>'
})
class App {}
bootstrap(App, [])

The error is

EXCEPTION: Template parse errors:
Can't bind to 'ngForIn' since it isn't a known native property
("<div [ERROR ->]*ngFor="let talk in talks">

Solution 1:

Since this is at least the third time I've wasted more than 5 min on this problem I figured I'd post the Q & A. I hope it helps someone else down the road... probably me!

I typed in instead of of in the ngFor expression.

Befor 2-beta.17, it should be:

<div *ngFor="#talk of talks">

As of beta.17, use the let syntax instead of #. See the UPDATE further down for more info.

Note that the ngFor syntax "desugars" into the following:

<template ngFor #talk [ngForOf]="talks">
  <div>...</div>
</template>

If we use in instead, it turns into

<template ngFor #talk [ngForIn]="talks">
  <div>...</div>
</template>

Since ngForIn isn't an attribute directive with an input property of the same name (like ngIf), Angular then tries to see if it is a (known native) property of the template element, and it isn't, hence the error.

UPDATE - as of 2-beta.17, use the let syntax instead of #. This updates to the following:

<div *ngFor="let talk of talks">

Note that the ngFor syntax "desugars" into the following:

<template ngFor let-talk [ngForOf]="talks">
  <div>...</div>
</template>

If we use in instead, it turns into

<template ngFor let-talk [ngForIn]="talks">
  <div>...</div>
</template>

Solution 2:

TL;DR;

Use let...of instead of let...in !!

If you're new to Angular (>2.x) and possibly migrating from Angular1.x, most likely you're confusing in with of. As andreas has mentioned in the comments below for ... of iterates over values of an object while for ... in iterates over properties in an object. This is a new feature introduced in ES2015.

Simply replace:

<!-- Iterate over properties (incorrect in our case here) -->
<div *ngFor="let talk in talks">

with

<!-- Iterate over values (correct way to use here) -->
<div *ngFor="let talk of talks">

So, you must replace in with of inside ngFor directive to get the values.

Solution 3:

There is an alternative if you want to use of and not switch to in. You can use KeyValuePipe introduced in 6.1. You can easily iterate over an object:

<div *ngFor="let item of object | keyvalue">
  {{item.key}}:{{item.value}}
</div>

Solution 4:

Try to import import { CommonModule } from '@angular/common'; in angular final as *ngFor ,*ngIf all are present in CommonModule

Solution 5:

use of instead of in. You can use KeyValue Pipe . You can easily iterate over an object:

<div *ngFor="let items of allObject | keyvalue">
  {{items.key}}:{{items.value}}
</div>